U.S. patent number 5,414,723 [Application Number 08/104,081] was granted by the patent office on 1995-05-09 for infrared laser system.
Invention is credited to Vladimir B. Krapchev.
United States Patent |
5,414,723 |
Krapchev |
* May 9, 1995 |
Infrared laser system
Abstract
An infrared laser system includes a neodymium laser for
generating a pulsed laser beam at a wavelength of 1.06 micrometers
and a Raman cell containing a Raman active medium. The laser beam,
having sufficient peak power to cause emission of light from the
Raman active medium by stimulated Raman scattering, is directed
through the Raman cell. Ethanol-d.sub.1 or methanol-d.sub.1 is used
as the Raman active medium to generate wavelengths of about 1.5
micrometers, 2.8-2.9 micrometers, or both. The laser is preferably
a neodymium YAG laser.
Inventors: |
Krapchev; Vladimir B.
(Brookline, MA) |
[*] Notice: |
The portion of the term of this patent
subsequent to October 6, 2009 has been disclaimed. |
Family
ID: |
24638028 |
Appl.
No.: |
08/104,081 |
Filed: |
August 11, 1993 |
PCT
Filed: |
February 14, 1992 |
PCT No.: |
PCT/US92/01230 |
371
Date: |
August 11, 1993 |
102(e)
Date: |
August 11, 1993 |
PCT
Pub. No.: |
WO92/15137 |
PCT
Pub. Date: |
September 03, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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657641 |
Feb 15, 1991 |
5153887 |
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Current U.S.
Class: |
372/3; 372/4;
372/51 |
Current CPC
Class: |
H01S
3/307 (20130101) |
Current International
Class: |
H01S
3/30 (20060101); H01S 003/30 () |
Field of
Search: |
;372/3,4,51 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
J G. Meadors et al, "Generation of Infrared Radiation . . ." J.
Quantum Electronics, vol. QE-8, No. 4, Apr. 1972, pp. 427-428.
.
A. V. Krumin'sh et al, Sov. J. Quantum Electron, vol. 14, No. 7,
Jul. 1984, pp. 1001-1002. .
C. Guntermann et al, Applied Optics, vol. 28, No. 1, 1 Jan. 1989,
pp. 135-138. .
E. Patterson et al, Applied Optics, vol. 28, No. 23, 1 Dec. 1989,
pp. 4978-4981. .
J. Hampton, "A Terrestrial Optical Scatter Communications Model",
1989, pp. 263-268. .
M. J. Colles, Optics Communications, vol. 1, No. 4, Sep./Oct. 1963,
pp. 169-172. .
D. C. Hanna et al, IEEE J. Quantum Electronics, vol. QE-22, No. 2,
Feb. 1986, pp. 332-336. .
T. R. Loree, IEEE J. Quantum Electronics, vol. QE-15, No. 5, May
1979, pp. 337-342. .
J. J. Ottusch et al, IEEE J. Quantum Electronics, vol. 24, No. 10,
Oct. 1988, pp. 2076-2080. .
"Tables of Molecular Vibrational Frequencies, v.1", Nat. Bureau of
Standards, 1972, pp. 63-66. .
C. J. Pouchert, "The Aldrich Library of IR Spectra"2nd Ed., 1978
(one sheet). .
D. Schiel et al, Chemical Physics Letters, vol. 166, No. 1, 9 Feb.
1990, pp. 82-86. .
S. J. Pfeifer, Proc. SPIE, vol. 1000, Laser Wavefront Control,
1988, pp. 33-42. .
J. Munch et al, Applied Optics, vol. 28, No. 15, 1 Aug. 1989, pp.
3099-3105. .
D. Stern et al, Opthamology, vol. 95, No. 10, Oct. 1988, pp.
1434-1441. .
M. Falk et al, J. Chemical Physics, vol. 34, No. 5, May 1961, pp.
1554-1568. .
Kaiser W., "Transient Stimulated Raman Scattering. Relaxation Times
of Molecular Vibrations", Sov. J. Quant. Electron., vol. 4, No. 9,
Mar. 1975, New York, pp. 1131-1134..
|
Primary Examiner: Davie; James W.
Attorney, Agent or Firm: Wolfe, Greenfield & Sacks
Parent Case Text
RELATED APPLICATION
This application is a continuation-in-part of U.S. application Ser.
No. 07/657,641 filed Feb. 15, 1991, now U.S. Pat. No. 5,153,887,
the entire disclosure of which is incorporated herein by reference.
Claims
What is claimed is:
1. An infrared laser system comprising:
a neodymium laser for generating a pulsed laser beam having a
wavelength of 1.06 micrometers;
a Raman cell containing a Raman active medium selected from the
group consisting of ethanol-d.sub.1 and methanol-d.sub.1 ; and
means for coupling said laser beam through said Raman cell, said
laser beam having sufficient power to cause emission of light from
said Raman active medium.
2. An infrared laser system as defined in claim 1 wherein said
neodymium laser has a pulse width of 100 picoseconds or less.
3. An infrared laser system as defined in claim 1 wherein said
laser comprises a mode-locked neodymium YAG laser.
4. An infrared laser system as defined in claim 1 wherein said
neodymium laser has a pulse width T.sub.p less than T.sub.B
(gG/4g.sub.B), where T.sub.B is the Brillouin lifetime, g is the
Raman gain coefficient, G is the total integrated gain and g.sub.B
is the Brillouin gain coefficient.
5. An infrared laser system as defined in claim 1 wherein said
neodymium laser comprises a broadband Q-switched neodymium laser
having a bandwidth .GAMMA..sub.B of 1 cm.sup.-1 or more.
6. An infrared laser system as defined in claim 1 wherein said
neodymium laser comprises a broadband Q-switched neodymium laser in
which the wideband Brillouin gain coefficient is smaller than the
Raman gain coefficient.
7. An infrared laser system as defined in claim 1 wherein said
Raman active medium comprises ethanol-d.sub.1 and wherein said
laser beam has sufficient power to cause emission of light from
said Raman active medium at 1.54 micrometers.
8. An infrared laser system as defined in claim 1 wherein said
Raman active medium comprises ethanol-d.sub.1 and said laser beam
has sufficient power to cause emission of light from said Raman
active medium at 2.79 micrometers.
9. An infrared laser system as defined in claim 1 wherein said
Raman active medium comprises methanol-d.sub.1 and wherein said
laser beam has sufficient power to cause emission of light from
said Raman active medium at 1.51 micrometers and 1.54
micrometers.
10. An infrared laser system as defined in claim 1 wherein said
Raman active medium comprises methanol-d.sub.1 and wherein said
laser beam has sufficient power to cause emission of light from
said Raman active medium at about 2.8-2.9 micrometers.
11. An infrared laser system as defined in claim 1 further
including means for increasing the viscosity of said Raman active
medium sufficiently to suppress Brillouin backscattering.
12. An infrared laser system as defined in claim 11 wherein said
means for increasing viscosity of said Raman active medium
comprises means for cooling said Raman active medium sufficiently
to increase the viscosity thereof.
13. An infrared laser system as defined in claim 12 wherein said
means for cooling is sufficient to reduce the temperature of said
Raman active medium to about -50.degree. C.
14. An infrared laser system as defined in claim 11 wherein said
means for increasing the viscosity of said Raman active medium
comprises a viscous material mixed with said Raman active
medium.
15. An infrared laser system as defined in claim 14 wherein said
viscous material comprises deuterated glycerol.
16. An infrared laser system as defined in claim 15 wherein said
deuterated glycerol comprises about 40% by weight of said Raman
active medium.
17. An infrared laser system as defined in claim 1 further
including a viscous material mixed with said Raman active
medium.
18. An infrared laser system as defined in claim 17 wherein said
viscous material comprises deuterated glycerol.
19. An infrared laser system as defined in claim 18 wherein said
deuterated glycerol has a concentration of about 20% to 60% by
weight of said Raman active medium.
20. An infrared laser system as defined in claim 18 wherein
said-deuterated glycerol has a concentration of about 40% by weight
of said Raman active medium.
21. An infrared laser system as defined in claim 1 wherein said
Raman cell includes an input window for receiving said laser beam,
an output window and means for containing said Raman active medium
between said input window and said output window, and said coupling
means includes a lens for focusing said laser beam in said Raman
active medium.
22. An infrared laser system as defined in claim 1 wherein said
coupling means includes a lens for focusing said laser beam in said
Raman active medium.
23. An infrared laser system as defined in claim 22 wherein said
coupling means further includes a polarizer and Faraday rotator for
isolating the laser beam from radiation that is backscattered from
said Raman cell.
24. An infrared laser system as defined in claim 23 wherein said
Raman cell further includes means for cooling said Raman active
medium.
25. An infrared laser system as defined in claim 22 wherein said
coupling means further includes means for isolating the laser from
radiation that is backscattered from said Raman cell.
26. An infrared laser system as defined in claim 1 further
including beam splitting means for separating said laser beam at
1.06 micrometers from the light emitted from said Raman active
medium.
27. An infrared laser system as defined in claim 1 further
including means for circulating said Raman active medium through
said Raman cell.
Description
FIELD OF THE INVENTION
This invention relates to laser systems for generating radiation at
wavelengths of about 1.5 micrometers and wavelengths of about
2.8-2.9 micrometers and, more particularly, to laser systems for
frequency shifting the light from a neodymium laser by stimulated
Raman scattering.
BACKGROUND OF THE INVENTION
A desirable wavelength for laser radar (lidar) and over-the-horizon
optical communications is 1.5 micrometers because this wavelength
is considered to be eyesafe. Wavelengths in the range of about
2.8-2.9 micrometers have been found useful for medical applications
such as laser surgery because these wavelengths are highly absorbed
by the water in tissue and thus are effective for vaporizing
tissue. However, reliable, low cost, high power lasers for directly
generating such wavelengths are not commercially available.
Pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers are
commercially available from a number of suppliers and are used in
many fields due to their relatively low cost, large average and
peak powers and high pulse repetition frequencies. A well-known
method for frequency shifting a laser toward longer wavelengths is
by stimulated Raman scattering. A laser beam is directed through a
Raman cell containing a Raman active medium. When the intensity of
the laser beam exceeds a threshold value, light is emitted by the
Raman medium at a wavelength that is longer than the wavelength of
the laser beam. The output of the Raman cell includes light at the
laser wavelength and at the shifted wavelength. The frequency shift
and the conversion efficiency are characteristics of the Raman
medium.
The use of methane for Raman shifting of a neodymium YAG laser
output at 1.06 micrometers to a wavelength of 2.8 micrometers is
disclosed by Guntermann et al in Applied Optics, Vol. 28, No. 1,
Jan. 1, 1989, pages 135-138. Medical applications are suggested.
The generation of 1.54 micrometer radiation for laser radar using
methane as a Raman active medium is disclosed by Patterson et al in
Applied Optics, Vol. 28, No. 23, Dec. 1, 1989, pages 4978-4981.
Deposition of soot-like particles on the Raman cell windows was
reported to limit the operating life of the system.
Raman scattering using methyl, ethyl and isopropyl alcohol,
acetone, trichloroethane and water as Raman active media is
disclosed by Colles in Optics Communications, Vol. 1, No. 4,
September/October 1969, pages 169-172. Picosecond pulses at 530
nanometers, which were provided by second harmonic generation of
the output of a neodymium glass laser, were used as the pump
pulses.
Efficient conversion of light from 30 picosecond pulses of a Nd:YAG
laser at 1.064 micrometers to the first Stokes component at
1.53-1.56 micrometers in cyclohexane, acetone, 1,2-dichloroethane
and 1,4-dioxane is disclosed by Krumin'sh et al in Soviet Journal
of Quantum Electronics, Vol. 14, No. 7, July 1984, pages
1001-1002.
Raman conversion in acetonitrile and methane to the eye-safe
wavelength near 1.5 micrometers from a Q-switched Nd:YAG laser is
disclosed by Meadors and Poirier in IEEE Journal of Quantum
Electronics, Vol. QE-8, No.4, April 1972, pages 427-428.
Stimulated Raman scattering of 100 picosecond pulses in hydrogen,
deuterium and methane is disclosed by Hanna et al in IEEE Journal
of Quantum Electronics, Vol. QE-22, No. 2, February 1986, pages
332-336. A mode-locked and Q-switched neodymium YAG laser followed
by a second harmonic generator was used to generate pump pulses at
1.06 micrometers and 0.53 micrometers. The use of hydrogen,
deuterium and methane as Raman active media are also disclosed by
Lorre et al in IEEE Journal of Quantum Electronics, Vol. QE-15, No.
5, May 1979, pages 337-342 and by Ottusch et al in IEEE Journal of
Quantum Electronics, Vol. 24, No. 10, October 1988, pages
2076-2080.
A Raman cell positioned inside a neodymium YAG laser resonant
cavity is disclosed in U.S. Pat. No. 4,327,337, issued Apr. 27,
1982 to Liu. Deuterium is suggested as a Raman active medium. A 1.5
micron Raman laser is disclosed in U.S. Pat. No. 3,668,420, issued
Jun. 6, 1972 to Vanderslice. A laser system for generating
radiation in the ultraviolet wavelength range using a plurality of
Raman cells is disclosed in U.S. Pat. No. 4,254,348, issued Mar. 3,
1981 to Stappaerts. Deuterium is disclosed as a Raman active
medium.
All-the known techniques for generation of radiation at 1.5
micrometers and 2.8-2.9 micrometers have been subject to one or
more problems, including a short operating life, low efficiency and
Brillouin backscattering from the Raman active medium. It is
desirable to provide laser systems which overcome these
problems.
It is a general object of the, present invention to provide
improved laser systems.
It is another object of the present invention to provide laser
systems for efficient Raman shifting of the 1.06 micrometer
radiation from a neodymium laser to about 1.5 micrometers.
It is a further object of the present invention to provide laser
systems for efficient Raman shifting of the 1.06 micrometer
radiation from a neodymium laser to about 2.8-2.9 micrometers.
It is another object of the present invention to provide reliable,
long life laser systems for generating radiation at about 1.5
micrometers and about 2.8-2.9 micrometers.
It is yet another object of the present invention to provide Raman
active media for efficient conversion of 1.06 micrometer radiation
to radiation at about 1.5 micrometers and about 2.8-2.9
micrometers.
SUMMARY OF THE INVENTION
According to the present invention, these and other objects and
advantages are achieved in an infrared laser system including a
neodymium laser for generating a pulsed laser beam at a wavelength
of 1.06 micrometers, a Raman cell containing a Raman active medium
and means for coupling the laser beam through the Raman cell, the
laser beam having sufficient peak power to cause emission of light
from the Raman active medium by stimulated Raman scattering.
Depending on the Raman active medium and the peak power of the
laser beam, the output of the laser system is at a wavelength of
about 1.5 micrometers, at a wavelength of about 2.8-2.9
micrometers, or both. The Raman active media in accordance with the
invention convert the laser beam at 1.06 micrometers to the desired
wavelengths with high efficiency.
The Raman active medium comprises ethanol-d.sub.1 which provides an
output wavelength of 1.54 micrometers, 2.79 micrometers, or both,
or methanol-d.sub.1 which provides output wavelengths of 1.51
micrometers and 1.54 micrometers, 2.8-2.9 micrometers, or both.
According to one feature of the invention, the laser comprises a
mode-locked neodymium YAG laser having a pulse width of 100
picoseconds or less.
According to another feature of the invention, the laser comprises
a broadband, Q-switched neodymium YAG laser having a pulse width of
one nanosecond or greater. The laser has a bandwidth .GAMMA..sub.B
greater than one cm.sup.-1.
According to a further feature of the invention, stimulated
Brillouin backscattering is reduced by increasing the viscosity of
the fluids to provide Raman conversion independent of the laser
pulselength and bandwidth. When ethanol-d.sub.1 and
methanol-d.sub.1 are mixed with deuterated glycerol (CH.sub.2
ODCHODCH.sub.2 OD), one finds that the solution is transparent at
the relevant wavelengths and can have a viscosity greater by a
factor of 10. In such a solution, the Raman scattering dominates
Brillouin backscattering. A similar effect can be achieved by
cooling the pure deuterated alcohols below -50.degree. C., but this
appears less attractive in practice.
In the laser system, the means for coupling the laser beam through
the Raman cell typically includes a lens for focusing the laser
beam in the Raman active medium. The coupling means preferably
further includes a polarizer and a Faraday rotator for isolating
the laser from radiation that is backscattered from the Raman
cell.
The Raman cell includes an input window for receiving the laser
beam, an output window and means for containing the Raman active
medium between the input window and the output window. The Raman
cell typically includes means for cooling the Raman active medium.
The Raman active medium can be circulated through the Raman
cell.
The laser system according to the invention typically includes an
output lens for collimating the output of the Raman cell. The
outputs of the Raman cell can be separated into individual
wavelengths by one or more dichroic beam splitters or prisms.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the present invention, together with
other and further objects, advantages and capabilities thereof,
reference is made to the accompanying drawings which are
incorporated herein by reference and in which:
FIG. 1 is a schematic diagram of an infrared laser system in
accordance with the present invention;
FIG. 2 is a graph showing the transmission spectrum of 1 cm of
methanol at wavelengths between 1.0 and 1.6 micrometers;
FIG. 3 is a graph showing the transmission spectrum of 1 cm of
methanol-d.sub.1 at wavelengths between 1.0 and 1.6
micrometers;
FIG. 4 is a graph showing the transmission spectrum of 1 cm of
ethanol-d.sub.1 at wavelengths between 1.0 and 1.6 micrometers;
FIG. 5 is a graph showing the transmission spectra of 1 mm of
ethanol-d.sub.1 at 99.8% and 99.9% purity at wavelengths between
2.6 and 2.9 micrometers; and
FIG. 6 is a graph showing the transmission spectra of 1 cm of 99.8%
ethanol-d.sub.1, and a solution of 99.8% ethanol-d.sub.1 and 98%
deuterated glycerol (CH.sub.2 ODCHODCH.sub.2 OD, 40% by weight in
the solution) at wavelengths between 1.0 and 1.6 micrometers.
DETAILED DESCRIPTION OF THE INVENTION
An infrared laser system in accordance with the present invention
is shown in FIG. 1. A laser 10 generates a laser beam 12 at a
wavelength of 1.06 micrometers (.mu.m). The laser 10 is a pulsed.
neodymium laser and is preferably a neodymium-doped yttrium
aluminum garnet (Nd:YAG) laser. A neodymium glass laser can also be
utilized when a relatively low pulse repetition rate is acceptable.
As described hereinafter, the laser 10 can be a Q-switched device
or a mode-locked device, depending on the required pulse width. The
laser beam 12 is directed through a polarizer 14, a Faraday rotator
16 and a lens 18 to a Raman cell 20. The Raman cell 20 includes a
cavity 22 that contains a generally cylindrical Raman active medium
in liquid form. The cavity 22 is defined by an input window 24, an
output window 26 and a generally cylindrical Raman cell wall
28.
The laser beam 12 is directed through the input window 24, the
Raman active medium and the output window 26. When the peak power
of the laser beam 12 is above a Raman threshold of the medium,
radiation is generated at a wavelength that is longer than the
wavelength of the laser beam 12 by stimulated Raman scattering. In
accordance with the present invention, Raman active media for
conversion of the laser beam at 1.06 micrometers to wavelengths of
about 1.5 micrometers, wavelengths of about 2.8-2.9 micrometers, or
both, are provided. The Raman active media are discussed in detail
hereinafter.
The polarizer 14 and the Faraday rotator 16 isolate the laser 10
from radiation that is backscattered from the Raman cell 20. Other
techniques for suppressing backscattered radiation are known in the
art. The laser beam 12 is focused in the Raman active medium by
lens 18. In cases where the peak power of the laser beam 12 is
sufficiently high, the lens 18 can be omitted, and a collimated
laser beam can be directed through the Raman cell 20. All the
optical elements in the laser system preferably include an
antireflection coating that is effective to reduce reflections at
1.06 micrometers.
The wall 28 of Raman cell 20 is preferably thermally conductive to
conduct heat away from the Raman active medium. An active cooling
system 30 can also be used for cooling the Raman cell 20. The
cooling system 30 can, for example, include a system for
circulating a cooling fluid such as water through the Raman cell
wall 28 for removal of heat generated in the Raman active medium. A
pump 32 can be used for circulating the Raman active medium through
the cavity 22.
An output beam 34 from the Raman cell 20 is directed through a lens
36 to a dichroic beam splitter 38. The output beam 34 is collimated
by lens 36 and is split into its component wavelengths by beam
splitter 38. The beam splitter 38 directs a first output beam
component 40 at 1.06 micrometers to a beam dump 42. A second output
beam component 44 includes radiation generated by the Raman active
medium. Depending on the Raman active medium utilized in Raman cell
20 and the peak power of laser beam 12, the second beam component
44 can include radiation at about 1.5 micrometers, radiation at
about 2.8-2.9 micrometers, or both. When the second beam component
44 contains radiation at about 1.5 micrometers and about: 2.8-2.9
micrometers, a prism 46 can be utilized to separate these
wavelengths. The prism 46 separates the second beam component 44
into a third beam component 48 at about 1.5 micrometers and a
fourth beam component 50 at about 2.8-2.9 micrometers.
The components of the laser system must be essentially transparent
to radiation at 1.06 micrometers. The input window 24 can, for
example, be fused silica. The output window 26 and the lens 36 must
be transparent to radiation in the wavelength range of about 1-3
micrometers. A suitable material for these components is magnesium
fluoride. A suitable material for the prism 46 is calcium
fluoride.
The generation by the Raman active medium of radiation at a
wavelength longer than the radiation of the laser beam 12 occurs as
a result of the process of stimulated Raman scattering. When the
peak power of the laser beam 12 exceeds a predetermined threshold,
the Raman active medium generates coherent radiation at a
wavelength that is a function of the input wavelength and the Raman
active medium. The frequency shift and the Raman gain coefficient
are characteristics of the medium. In accordance with the present
invention, liquid ethanol-d.sub.1 (CH.sub.3 CH.sub.2 OD) or liquid
methanol-d.sub.1 (CH.sub.3 OD) is used to generate wavelengths of
about 1.5 micrometers. Further in accordance with the invention,
liquid ethanol-d.sub.1 or liquid methanol-d.sub.1 is used to
generate wavelengths of about 2.8-2.9 micrometers. Details
regarding the Raman active media and the characteristics of the
laser 10 are described below.
The threshold peak power of laser beam 12 that is required for
stimulated Raman scattering in the focused geometry shown in FIG. 1
is given by:
where G is the total integrated gain, P is the laser peak power, g
is the Raman gain coefficient, .lambda. is the wavelength of laser
beam 12 (1.06 micrometers), l is the length of the Raman cell 20
along the optical beam path, f is the focal length of lens 18, d is
the diameter of laser beam 12 at the lens 18, and N is the ratio of
the area in the focal spot to the area of a diffraction limited
beam. In order to determine the threshold power P, the total gain G
is set at 30. The Raman gain coefficient g for each Raman active
medium is specified in Table I below. The remaining parameters are
determined from the geometry of the laser system. As an example,
for d=0.3 cm, f=1=10 cm and N=1, the threshold peak power P for
conversion to 1.54 micrometers of a neodymium YAG laser in
ethanol-d.sub.1 is 0.56 megawatt. For a Q-switched laser having a
pulse width of 5 nanoseconds, this corresponds to 2.8 millijoules
(mJ) per pulse. The power requirement for the generation of second
Stokes light at 2.8 micrometers is three times larger.
The properties of the Raman active media in accordance with the
present invention are summarized in Table I below. In Table I, g
represents the Raman gain coefficient, T.sub.2 represents the Raman
lifetime, g.sub.B represents the Brillouin gain coefficient and
T.sub.B represents the Brillouin lifetime. The data in Table I is
taken from published literature.
TABLE I ______________________________________ Raman g.sub.B Shift
g(cm/GW) at T.sub.2 (cm/ T.sub.B Medium (cm.sup.-1) .lambda..sub.s
= 1.5 .mu.m (psec) GW) (nsec)
______________________________________ CH.sub.3 CH.sub.2 OH 2928
1.8 2 12 2.8 CH.sub.3 CH.sub.2 OD 2928* 1.8* 2* 12* 2.8* CH.sub.3
OH 2834 0.8 2 13 4 2944 0.6 3 CH.sub.3 OD 2834* 0.8* 2* 13* 4*
2944* 0.6* 3* ______________________________________ *Results from
measurements in d.sub.1 alcohols are unknown, but the Raman
scattering and Brillouin backscattering properties are expected to
be the same as those of ordinary alcohols.
According to a first embodiment of the invention, ethanol-d.sub.1
and methanol-d.sub.1 are used for Raman conversion of a neodymium
YAG laser at 1.06 micrometers to the first Stokes wavelengths at
1.54 micrometers (ethanol-d.sub.1) or 1.51 and 1.54 micrometers
(methanol-d.sub.1). The primary difference between alcohols and
deuterated d.sub.1 alcohols is a shift of the OH stretch frequency
to an OD frequency and a dramatic increase in transmission at the
relevant wavelengths without a change in the Raman properties due
to the CH.sub.3 stretch mode. In FIG. 2, the transmission spectrum
of 1 cm of methanol, indicated by curve 60, shows large absorption
near 1.5 micrometers. The same result was found for ethanol. By
comparison, the spectra of methanol-d.sub.1 (curve 62 in FIG. 3)
and ethanol-d.sub.1 (curve 64 in FIG. 4) show transmission in
excess of 80% near 1.5 micrometers. This result makes them suitable
Raman liquids. The spectra shown in FIGS. 2-6 were obtained on a
Cary photospectrometer.
According to a second embodiment of the invention, ethanol-d.sub.1
and methanol-d.sub.1 are used for conversion of laser radiation at
1.06 micrometers to the second Stokes light near 2.8 micrometers.
Because of the two first Stokes components, methanol-d.sub.1 is
believed to produce up to four second Stokes wavelengths at a
significantly reduced efficiency and is therefore less preferred
than ethanol-d.sub.1. As the absorption spectra of 1 mm
ethanol-d.sub.1 in FIG. 5 indicate, the results are very sensitive
to the presence of impurities. Curve 66 is the spectrum of
ethanol-d.sub.1 at 99.8% purity, and curve 68 is the spectrum of
ethanol-d.sub.1 at 99.9% purity. The major contaminant is water
with an absorption coefficient, .alpha..sub.w =10.sup.4 cm.sup.-1,
at these wavelengths. The results are consistent with the presence
of 0.1% water in the ethanol-d.sub.1, and the absorption
coefficient is on the order of .alpha..sub.a =10 cm.sup.-1. In
order to overcome the absorption, one requires gI>.alpha..sub.a,
where g is the Raman gain coefficient at 2.8 .mu.m and I is the
light intensity at 1.5 .mu.m. This leads to intensities greater
than 10 GW/cm.sup.2. Under these conditions, the second Stokes
light can increase exponentially and may saturate the contaminant
water absorption. For laser pulses shorter than the water
relaxation time of T.sub.r =20 picoseconds (psec), the water
absorption .alpha..sub.w decreases according to
where F is the laser fluence, F.sub.s =n.omega./2.sigma..sub.a is
the saturation fluence, n is the Planck constant, .omega. is the
radian frequency at 2.8 .mu.m and .sigma..sub.a is the water
absorption cross-section at the same wavelength. With .sigma..sub.a
=3.times.10.sup.-19 cm.sup.2, one finds F.sub.s =0.1 J/cm.sup.2.
The output laser fluence, F, at 2.8 .mu.m should exceed this value
by at least an order of magnitude. For laser pulses longer than 20
psec, the water absorption decreases according to
where I is the laser intensity at 2.8 .mu.m and I.sub.s
=n.omega./2.sigma..sub.a T.sub.r is the saturation intensity. The
value of I.sub.s for water near 2.8 .mu.m is estimated at I.sub.s
=5 GW/cm.sup.2. Output intensities in excess of 50 GW/cm.sup.2 will
be required. This can be achieved with pulselengths of 100 psec or
less, but not with nanosecond pulses. The required intensity is
believed to be below that for optical breakdown. Clearly,
mode-locked Nd:YAG lasers are the preferred system for generation
of Raman shifted 2.8 .mu.m radiation.
One problem with the operation of Raman cells is Brillouin
backscattering in which the laser beam is reflected by the Raman
active medium and stimulated Raman scattering is suppressed or does
not occur. Brillouin backscattering is virtually eliminated at
short laser pulses by making it highly transient. The requirement
for dominance of steady state Raman scattering over transient
Brillouin backscattering can be expressed in terms of the laser
pulselength, T.sub.p,
where the parameters of the right hand side of equation (3) are
defined in connection with Table I and equation (1).
In accordance with another feature of the invention, the Raman
active medium in Raman cell 20 comprises liquid ethanol-d.sub.1 or
liquid methanol-d.sub.1 and the laser 10 comprises a neodymium YAG
laser with a pulse length T.sub.p given by equation (3). For
conversion to 1.54 .mu.m in ethanol-d.sub.1 with G=50, the laser
pulselength is less than 5 nsec. For conversion to 1.51 and 1.54
.mu.m in methanol-d.sub.1, the pulselength is less than 2 nsec. For
conversion to second Stokes at 2.79 .mu.m in ethanol-d.sub.1, the
Raman gain coefficient is reduced by a factor of 2 and the
integrated gain is based on the peak power at the first Stokes
radiation. With a G=50, one finds a laser pulselength of less than
2.5 nsec. The preferred laser 10 is a mode-locked neodymium YAG
laser having a pulselength of 100 psec or less.
According to a further feature of the invention, the Brillouin
backscattering can be reduced by the use of wideband lasers.
Typical Q-switched neodymium YAG lasers have many longitudinal
modes. The spacing between the modes, .OMEGA., depends on the laser
cavity and varies from 100 MHz to a few GHz. A laser bandwidth of 1
cm.sup.-1 corresponds to 30 GHz and the spectrum may contain 10-100
longitudinal modes. The Brillouin gain coefficient for wideband
lasers, g.sub.B (WB) is related to that for a single longitudinal
mode (narrowband laser), g.sub.B, by the equation ##EQU1## where
g.sub.B is given in Table I, .GAMMA..sub.B =T.sub.B.sup.-1 is the
Brillouin bandwidth, M is the number of longitudinal modes and
.omega..sub.B is the Brillouin frequency shift. For the alcohols,
.omega..sub.B is of the order of 4-5 GHz. A typical reduction in
the wideband gain coefficient is g.sub.B (WB)=g.sub.B /M. Raman
scattering will dominate Brillouin backscattering when g>g.sub.B
(WB).
According to yet another feature of the invention, the Brillouin
backscattering can be reduced by increasing the viscosity, .eta.,
of the Raman medium. The Brillouin gain coefficient is inversely
proportional to the viscosity, g.sub.B .varies..eta..sup.-1. A
decrease in temperature to -50.degree. C. increases the viscosity
of ethanol by a factor of 6, which is sufficient to suppress
Brillouin backscattering.
A more promising approach is to add to the alcohols a highly
viscous fluid ,without changing the absorption at the relevant
wavelengths. We have found such a solution, which represents
another feature of the invention. FIG. 6 shows the absorption
spectrum of pure ethanol-d.sub.1 (indicated by curve 70) and a
solution of ethanol-d.sub.1 and 40% by weight deuterated glycerol
(CH.sub.2 ODCHODCH.sub.2 OD) (indicated by curve 72). The solution
has absorption properties similar to ethanol-d.sub.1 but its
viscosity is greater by a factor of 4. In this case, the Brillouin
backscattering is reduced by a factor of 4, while Raman scattering
is reduced only by 40%. The concentration of deuterated glycerol in
ethanol-d.sub.1 or methanol-d.sub.1 is preferably in a range of
about 20% to 60% by weight to provide sufficient viscosity to
suppress Brillouin backscattering. Another feature of the invention
is to use viscous solutions of ethanol-d.sub.1 and methanol-d.sub.1
to suppress Brillouin scattering and Raman shift the laser light
without any constraints on the laser pulse length and
bandwidth.
The quantum efficiency of Raman conversion is given by the ratio of
the laser wavelength to the Stokes wavelength. With a pump laser
beam of 1.06 micrometers, the quantum efficiency for generation of
a first Stokes component at 1.5 micrometers is 67%, and for the
generation of a second Stokes component at 2.8-2.9 micrometers the
quantum efficiency is 36-34%. Based on experiments with deuterium
at excimer laser wavelengths and alcohols with frequency doubled
neodymium YAG lasers, one can expect Raman conversion in excess of
50% of the quantum efficiency. The estimated power conversion
efficiencies with the present invention are at least 50% to obtain
radiation at 1.5 micrometers and at least 20% for radiation at
2.8-2.9 micrometers. This compares very favorably with previously
reported results of 3% conversion efficiency to 2.8 micrometers
using methane (CH.sub.4) as the Raman medium.
While there have been shown and described what are at present
considered the preferred embodiments of the present invention, it
will be obvious to those skilled in the art that various changes
and modifications may be made therein without departing from the
scope of the invention as defined by the appended claims.
* * * * *